This invention relates to a method of time division multiplexing input signals into an output signal having a frame period which defines successive frames of the output signal. This invention relates also to a device for use in carrying out the method.
It is possible to understand that the input signals are received through first through N-th channels, where N represents an integer greater than one. In this event, the input signals are identified by first through N-th channel numbers.
In general, the input signals have a common bit rate. The common bit rate is, for example, 64 kbps (kilobits per second) when each of the input signals is a pulse code modulated (PCM) signal for a speech signal and when the integer N is equal to twenty-four. In this case, the output signal has a bit rate of 1.544 Mbps (megabits per second). First through N-th samples are extracted in the frame period from the respective input signals at a unit interval corresponding to the common bit rate. The first through the N-th samples are arranged in first through N-th fields in each of the frames. The first through the N-th fields have a time duration in common.
Thus, the first through the N-th fields are exclusively or fixedly assigned to the input signals of the first through the N-th channel numbers, respectively.
The first through the N-th samples are classifiable into significant and insignificant samples in each of the frames. The significant samples are obtained from the first through the N-th input signals when a significant signal is present in each of the first through the N-th channels. The significant signal is, for example, the speech signal, a data signal, and a video signal when the first through the N-th channels are telephone channels, data channels, and video channels, respectively. The significant signal is never continuously present. That is, each pause is interposed between the significant signals. The insignificant samples are obtained within the pauses.
When a person speaks, the pauses are present in the speech. It is statistically known in the art that the pauses last in total about 60 percent of a total duration of the speech. This likewise applies also when the significant signal is the data signal or the video signal.
The output signal is transmitted to a digital transmission line. In order to effectively utilize the digital transmission line, the significant samples may be transmitted to the digital transmission line as the output signal with preference to the insignificant samples. In accordance with this method, it is possible to transmit the input signals of as much as one hundred telephone channels while the input signals of only forty-eight channels are transmitted when all the significant and the insignificant samples are dealt with.
A method of dealing with the significant samples with preference to the insignificant samples, is described in an article contributed by Rikio MARUTA et al to IEEE Transactions On Communications, Vol. COM-29, No. 3 (March 1981), pages 337-345, under the title of "Design and Performance of a DSI Terminal for Domestic Applications." It is to be noted that the Maruta et al article refers to only a case where the input signals have the common bit rate. In other words, the Maruta et al article does not disclose an application to another case where the input signals have different bit rates which will herein be called first through M-th bit rates, where M represents a natural number which is not greater than the integer N. It should also be noted for such different rates that the samples of the respective input signals would have to be extracted at different unit intervals or periods, which may be referred to as first through M-th unit intervals and be understood to be in one-to-one correspondence to the first through the M-th bit rates.
A time division multiplexing method for the input signals of different bit rates is described in a paper contributed by Yutaka MORIYAMA et al to IEEE Global Telecommunication Conference, CH2190-7 (1985), pages 552-556, under the title of "Digital Multimedia Information Multiplexer." With the Moriyama et al paper, it is impossible to effectively utilize the digital transmission line. This is because all of the significant samples and the insignificant samples are transmitted to the digital transmission line without preference to the insignificant samples.